1. Field of the Invention
This invention relates to a negative-ion source, and more particularly to a negative-ion source for generating negative-ion beams by alkaline metal ion sputtering.
2. Prior Art
As a known negative-ion source for generating negative ions by causing alkaline metal ion particles to collide against negative-ion source seed material in the presence of neutral alkaline metal particles, that is, for generating negative ions by alkaline metal ion sputtering, the disclosure of apparatus has already been made in the "NUCLEAR INSTRUMENTS AND METHODS 144(1977) (USA) pp. 373-399" and "NUCLEAR INSTRUMENTS AND METHODS 164 (1979) (USA) pp. 4-10".
The former apparatus is so constructed that it employs negative ion seed material made to stick in the form of a cone to the perimeter of an aperture made in a target plate and uses cesium as alkaline metal. It thereby accelerates, by means of an ion extraction electrode, cesium ion particles discharged from a cesium ion discharging assembly to make the particles collide against the negative-ion seed material; on the other hand, neutral cesium particles are supplied to the negative-ion seed material from a cesium oven installed independently of the above structure. The negative ions generated are extracted from the aperture of the target plate in a direction opposite to the cesium ion discharge assembly and accelerated by the negative-ion extraction electrode to form negative-ion beams. In the case of this apparatus, the advantage is that sputtering efficiency is quite high because cesium ion particles are accelerated, before being allowed to collide against the negative-ion seed material. However, it has this disadvantage, that it tends to be larger because the ion discharge assembly which is to provide cesium ions after accelerating them and the ion extraction electrode (these are used as the ion source of cesium ions) are independent of the cesium oven. Moreover, although it is possible to discharge neutral cesium particles as well as cesium ion particles from the cesium ion discharge assembly, the quantity of neutral cesium particles proceeding toward the target plate from the cesium ion discharge assembly is decreased because the ion extraction and suppressor electrodes stand between the cesium ion discharge assembly and the target plate and because the distance between the cesium ion discharge assembly and the target plate tends to become greater. Accordingly, the omission of the cesium oven is impossible.
On the other hand, the latter apparatus employs means for discharging ions in such a way as to supply neutral cesium particles to an ion discharge assembly for the purpose of generating cesium ion particles. negative-ion seed material is arranged in the ion discharge assembly. Cesium ion particles are made to collide with the negative-ion seed material immediately after being generated, then negative-ion particles are generated. Consequently, negative-ion particles are directly extracted from the ion discharge assembly. For this apparatus, the advantage is that its construction is free from complexity and can be made compact because it is unnecessary to separately provide a cesium oven. However, this apparatus is at a disadvantage in being unable to accelerate cesium ion particles sufficiently, thereby reducing the sputtering efficiency. In addition, another disadvantage is that the number of negative-ion particles that may be extracted is decreased because, when the generated negative-ion particles are being extracted through the plasma in the ion discharge assembly, some of them are destroyed.